The present invention relates to a method for converting a certain type of energy to a different type of energy. For example, a method for generating electric energy from a temperature difference and vice versa or a method for converting light energy into heat energy. More particularly, the present invention relates to a method for converting between heat energy and electric energy by employing a carbon intercalation compound as a thermoelectric material, and employing a light-heat converting material to heat a fluid flowing in a hollow tube, wherein a thin carbon layer having metallic luster is accumulated on the inner surface thereof. The present invention also relates to a method for producing such thermoelectric material as well as a method for producing such a light-heat converting material.
It is well known that a figure of merit Z is the value for a thermoelectric performance representing a conversion efficiency between heat energy and electric energy. Z-value is estimated by a formula, Z=.alpha..sup.2 /.rho..kappa., where .alpha. is a Seebeck coefficient or a thermoelectric power (.mu.VK.sup.-1) , .rho. is an electrical resistivity (.OMEGA.cm) and .kappa. is a thermal conductivity (Wcm.sup.-1 K.sup.-1). Accordingly, it is obvious that a large .alpha., a small .rho. and a small .kappa. are required for obtaining a large figure of merit Z. On the other hand, in addition to the above conditions, the thermoelectric material is required to have a reliable PN controllability of its own. Furthermore, when used as a thermoelectric generator, the material is required to be operable at high temperatures so as to achieve high thermodynamic efficiency, or when used as a thermoelectric refrigerator, the material is required to realize PN-junction with high heat insulating properties.
Conventionally, various compounds, such as single-crystalline silicon semiconductor, bismuth or antimony chalcogenides, polycrystalline transition-metal silicides, amorphous bismuth chalcogenides or the like, are known as typical thermoelectric materials. However, among graphite intercalation compounds, only one has been found to be used as a thermoelectric material and the material is carbon fiber. Carbon fiber is used as a thermoelectric generator utilizing a temperature difference in the axial direction of fibers wherein an alkali metal or bromine is inserted between carbon layers of carbon fiber. There have been, however, two drawbacks in this practice. The first drawback is that, since the axial direction of fibers is parallel to the in-plane direction of a graphite structure, not only the electrical conductivity but also the thermal conductivity is large. Accordingly, inexhaustible high and low heat sources are required for thermoelectric generation, and this results in limited utilization for such a type of thermoelectric material. The second drawback is that, since alkali metal (potassium) or halogen (bromine) which have high reactivity, is employed as the intercalant for the graphite intercalation compound, the thermoelectric material is easily deteriorated by moisture or by oxygen in the air.
Subsequently, in regards to the light-heat converting material, it is well known that upon collection of radiant light by a metal reflected plate or a fluid, such as, water or Freon (trade name) gas in a glass tube which is arranged at a position where light is collected, and each is heated. It is more effective to provide the light-heat converting material on the inner surface of the glass tube which comes in contact with the fluid, than on the outer surface thereof, for effectively converting collected light to heat. A soot-like carbon material can be employed for the light-heat converting material in this case, because carbon is superior in light-heat conversion performance and is chemically and thermally stable. Furthermore, carbon has an extremely large thermal conductivity as compared with glass. There has been, however, a drawback in that the soot-like carbon deposit tends to be peeled from the glass tube due to the mechanical friction between the carbon deposit and the fluid flowing in the tube and as a result, it has not been practical to use.